Remote Automation Solutions Bristol 33XX Site Considerations for Equipment Owner's manual

Brand: Remote Automation Solutions

Model: Bristol 33XX Site Considerations for Equipment

Type: Owner's manual

SITE CONSIDERATIONS

For

EQUIPMENT INSTALLATION,

GROUNDING

WIRING

Bristol Babcock

Supplement Guide - S1400

Issue: 09/01

A Guide for the Protection of

Site Equipment & Personnel

In the Installation of

BBI Series 33XX DPCs & RTUs

NOTICE

The information in this document is subject to change without notice. Every effort has been

made to supply complete and accurate information. However, Bristol Babcock assumes no

responsibility for any errors that may appear in this document.

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Trademarks or copy-righted products mentioned in this document are for information only,

and belong to their respective companies, or trademark holders.

this manual may be reproduced in any form without the express written permission of

Bristol Babcock.

Supplement S1400 Page 0-1 Table Of Contents

Supplement Guide S1400

SITE CONSIDERATIONS FOR EQUIPMENT

INSTALLATION, GROUNDING & WIRING

TABLE OF CONTENTS

SECTION TITLE PAGE #

Section 1 - INTRODUCTION

1.1 GENERAL INTRODUCTION .......................................................................................1-1

1.2 MAJOR TOPICS ............................................................................................................. 1-1

Section 2 - PROTECTION

2.1 PROTECTING INSTRUMENT SYSTEMS................................................................... 2-1

2.1.1 Quality Is Conformance To Requirements.................................................................... 2-1

2.2 PROTECTING EQUIPMENT & PERSONNEL ........................................................... 2-1

2.2.1 Considerations For The Protection of Personnel ..........................................................2-2

2.2.2 Considerations For The Protection of Equipment ........................................................2-2

2.3 OTHER SITE SAFETY CONSIDERATIONS............................................................... 2-3

Section 3 - GROUNDING & ISOLATION

3.1 POWER & GROUND SYSTEMS................................................................................... 3-1

3.2 IMPORTANCE OF GOOD GROUNDS......................................................................... 3-1

3.3 transients and interference............................................................................................ 3-2

3.4 EARTH GROUND CONNECTIONS.............................................................................3-2

3.4.1 Establishing a Good Earth Ground. .............................................................................. 3-2

3.4.1.1 Soil Conditions ................................................................................................................3-4

3.4.1.2 Soil Types ........................................................................................................................3-4

3.4.1.3 Dry, Sandy or Rocky Soil................................................................................................ 3-6

3.5 GENERAL RECOMMENDATIONS ............................................................................. 3-7

3.5.1 Noise and Signal Errors ................................................................................................. 3-7

3.5.2 General Considerations.................................................................................................. 3-8

3.5.3 Other Grounding Considerations. ................................................................................. 3-8

3.6 GROUNDING TECHNIQUES FOR SERIES 33XX SYSTEMS ................................3-12

3.6.1 Several DPCs Mounted in Metal Cabinet with Power Supply ..................................3-12

3.6.2 Multiple DPC Cabinets with Local Power Supply in Each Cabinet..........................3-14

3.6.3 Multiple DPC Cabinets Powered by Single Power Supply ........................................3-14

3.6.4 Multiple Clusters of DPC Cabinets Powered by Local Supplies................................ 3-16

Section 4 - LIGHTNING ARRESTERS & SURGE PROTECTORS

4.1 STROKES & STRIKES .................................................................................................. 4-1

4.1.1 Chance of Being Struck by Lightning. ..........................................................................4-1

4.1.2 Antenna Caution ............................................................................................................4-3

4.1.3 Ground Propagation ....................................................................................................... 4-5

4.1.4 Tying it all Together.......................................................................................................4-5

4.1.5 Impulse Protection Summary ........................................................................................ 4-5

4.2 USE OF LIGHTNING ARRESTERS & SURGE PROTECTORS................................ 4-6

Supplement S1400 Page 0-2 Table Of Contents

Supplement Guide S1400

SITE CONSIDERATIONS FOR EQUIPMENT

INSTALLATION, GROUNDING & WIRING

TABLE OF CONTENTS

SECTION TITLE ..................................................................................................... PAGE #

Section 5 - WIRING TECHNIQUES

5.1 OVERVIEW ....................................................................................................................5-1

5.2 INSTRUMENT WIRING. ..............................................................................................5-1

5.2.1 Common Returns............................................................................................................5-1

5.2.2 Use of Twisted Shielded Pair Wiring (with Overall Insulation).................................. 5-2

5.2.3 Grounding of Cable Shields. .......................................................................................... 5-3

5.2.4 Use of Known Good Earth Grounds .............................................................................. 5-3

5.2.5 Earth Ground Wires ....................................................................................................... 5-3

5.2.6 Working Neatly & Professionally .................................................................................. 5-3

5.2.7 High Power Conductors and Signal Wiring ..................................................................5-4

5.2.8 Use of Proper Wire Size .................................................................................................5-4

5.2.9 Lightning Arresters & Surge Protectors ....................................................................... 5-4

5.2.10 Secure Wiring Connections............................................................................................ 5-5

REFERENCE DOCUMENTS

1. IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems - ANSI/IEEE Std

142-1982

2. IEEE Guide for the Installation of Electrical Equipment to Minimize Electrical Noise inputs to Controllers

from External Sources - IEE Std 518-1982

3. Lightning Strike Protect; Roy B. Carpenter, Jr. & Mark N. Drabkin, Ph.D.; Lightning Eliminators &

Consultant, Inc., 6687 Arapahoe Road, Boulder Colorado

4. Lightning Protection Manual for Rural Electric Systems, NRECA Research Project 82-5, Washington DC,

1983

5. Grounding for the Control of EMI; Hugh W. Denny; Don White Consultants, Inc., 1983, 1

Edition

6. Fundamentals of EGM - Electrical Installations; Michael D. Price; NorAm Gas Transmission, 525 Milam

Street, Shreveport, Louisiana 71151

Section 1 - Overview Page 1-1 S1400

Section 1 - Overview

1.1 INTRODUCTION

This document provides information pertaining to the installation of BBI Series 33XX

systems; more specifically, information covering reasons, theory and techniques for

protecting your personnel and equipment from electrical damage. Your instrument system

affects the quality of service provided by your company and many aspects of its operational

safety. Loss of instruments means lost production and profits as well as increased expenses.

Information contained in this document is for educational purposes. Bristol Babcock makes

no warranties or guarantees on the effectiveness or the safety of techniques described herein.

Where the safety of installations and personnel is concerned, refer to the National Electrical

Code Rules and rules of local regulatory agencies.

1.2 MAJOR TOPICS

Topics are covered in seven sections designed to pinpoint major areas of concern for the

protection of site equipment and personnel. The following overview is provided for each of

the major sections.

· Section 2 - Protection

This section provides the reasons for protecting instrument systems. An overview of the

definition of quality and what we are trying to accomplish in the protection of site

installations and how to satisfy the defined requirements is presented. Additionally,

this section provides considerations for the protection of personnel and equipment.

· Section 3 - Grounding & Isolation

Information pertaining to what constitutes a good earth ground, how to test and

establish such grounds, as well as when and how to connect equipment to earth grounds

is provided

· Section 4 - Lightning Arresters & Surge Protectors

Some interesting information dealing with Lightning strikes and strokes is presented in

technical and statistical form along with a discussion of how to determine the likelyhood

of a lightning strike.

Protecting equipment and personnel during the installation of

radios and antenna is discussed in a review of the dangers to equipment and personnel

when working with antennas. Reasons for the use of lightning arresters and surge

protectors are presented along with overviews of how each device protects site

equipment.

· Section 5 - Wiring Techniques

Installation of Power and “Measurement & Control” wiring is discussed. Information on

obscure problems, circulating ground and power loops, bad relays, etc. is presented.

Good wire preparation and connection techniques along with problems to avoid are

discussed. This sections list the ten rules of instrument wiring.”

Section 2 - Protection Page 2-1 S1400

Section 2 - Protection

2.1 PROTECTING INSTRUMENT SYSTEMS

Electrical instrumentation is susceptible to damage from a variety of natural and man

made phenomena. In addition to wind, rain and fire, the most common types of system and

equipment damaging phenomena are lightning, power faults, communication surges &

noise and other electrical interference’s caused by devices such as radios, welders,

switching gear, automobiles, etc. Additionally there are problems induced by geophysical

electrical potential & noise plus things that are often beyond our wildest imagination.

2.1.1 Quality Is Conformance To Requirements

A quality instrumentation system is one that works reliably, safely and as purported by the

equipment manufacturer (and in some cases by the system integrator) as a result of good

equipment design and well defined and followed installation practices. If we except the

general definition of quality to be, “quality is conformance to requirements,” we must also

except the premise that a condition of “quality” can’t exist where requirements for such an

end have not been evolved. In other words, you can’t have quality unless you have

requirements that have been followed. By understanding the requirements for a safe, sound

and reliable instrumentation system, and by following good installation practices (as

associated with the personnel and equipment in question), the operational integrity of the

equipment and system will be enhanced.

Understanding what is required to properly install BBI equipment in various en-

vironments, safely, and in accordance with good grounding, isolating and equipment

protection practices goes a long way toward maintaining a system which is healthy to the

owner and customer alike. Properly installed equipment is easier to maintain and operate,

and is more efficient and as such more profitable to our customers. Following good in-

stallation practices will minimize injury, equipment failure and the customer frustrations

that accompany failing and poorly operating equipment (of even the finest design). Ad-

ditionally, personnel involved in the installation of a piece of equipment add to or subtract

from the reliability of a system by a degree which is commensurate with their technical

prowess, i.e., their understanding of the equipment, site conditions and the requirements

for a quality installation.

2.2 PROTECTING EQUIPMENT & PERSONNEL

Series 33XX installations must be performed in accordance with National Electrical Code

Rules, electrical rules set by local regulatory agencies, and depending on the customer

environment (gas, water, etc), other national, state and local agencies such as the American

Water Works Association (AWWA). Additionally, installation at various customer sites may

be performed in conjunction with a “safety manager” or utility personnel with HAZMAT

(hazardous material) training on materials present (or potentially present) as required by

OSHA, the customer, etc.

S1400 Page 2-2 Section 2 - Protection

2.2.1 Considerations For The Protection of Personnel

Always evaluate the site environment as if your life depended on it. Make sure that you

understand the physical nature of the location where you will be working. Table 2-1

provides a general guideline for evaluating an installation site.

Table 2-1 - Installation Site Safety Evaluation Guide

# Guide

1 Indoor or outdoor – Dress Appropriately

2 If outdoor, what kind of environment, terrain, etc. Watch out for local varmint (bees,

spiders, snakes, etc.)

3 If indoor or outdoor – determine if there are any pieces of dangerous equipment or any

processes which might be a risk to your safety

4 If in a tunnel, bunker, etc. watch out for a build up of toxic or flammable gases. Make

sure the air is good. Watch out for local varmint (bees, spiders, snakes, etc.)

5 Hazardous or Non-Hazardous Environment – Wear appropriate safety equipment and

perform all necessary safety measures.

6 Before installing any equipment or power or ground wiring, make sure that there are no

lethal (life threatening) voltages between the site where the instrument will be installed

and other equipment, pipes, cabinets, etc. or to earth itself.

7 Never assume that adjacent or peripheral equipment has been properly installed and

grounded. Determine if this equipment and the 33XX unit in question can be touched

simultaneously without hazard to personnel and/or equipment?

8 Before embarking to remote locations where there are few or no human inhabitants ask a

few simple questions like, should I bring water, food, hygienic materials, first aid kit, etc?

Be Prepared!

9 Observe the work habits of those around you – for your own safety!

Some of the items that a service person should consider before ever going on site can be

ascertained by simply asking questions of the appropriate individual. Obviously other

safety considerations can only be established at the installation site.

2.2.2 Considerations For The Protection of Equipment

Always evaluate the site installation/service environment and equipment. Understand the

various physical interfaces you will be dealing with such as equipment mounting and

supporting, analog and digital circuits, power circuits, communication circuits and various

electrical grounds. Table 2-2 provides a general guideline for evaluating the equipment

protection requirements of an installation site.

Table 2-2 - Equipment Protection Site Safety Evaluation Guide

# Guide Reference Section

1 Environment - Class I, Division 2 - Nonincendive

Environment - Class I, Division 1 - Intrinsically Safe

Other - Safe or unrated area

See Appendix A of CI Manual

See Appendix B of CI Manual

2 Earth Ground - Established by mechanical/electrical or

(both) or not at all.

See Section 3

3 Is the area prone to lightning strikes? See Section 4

4 Are there surge suppressors installed or to be installed? See Section 4

5 Are there overhead or underground power or com-

munication cables in the immediate area?

See Section 2.3

Section 2 - Protection Page 2-3 S1400

Table 2-2 - Equipment Protection Site Safety Evaluation Guide (Continued)

# Guide Reference Section

6 Is there an antenna in the immediate area? See Section 4.1.2

7 How close is other equipment? Can someone safely touch this

equipment and a 33XX unit simultaneously?

See Section 2.3

8 Determine equipment ground requirements. How will the 33XX unit

and its related wiring be grounded? Consider Earth Ground, Circuit

Ground, Conduit Ground, Site Grounds!

See Section 3

9 Are there any obviously faulty or questionable power or ground

circuits?

See Section 2.3

2.3 OTHER SITE SAFETY CONSIDERATIONS

Overhead or underground power or communication cables must be identified prior to

installing a new unit. Accidentally cutting, shorting or simply just contacting power,

ground, communication or process control I/O wiring can have potentially devastating

effects on site equipment, the process system and or personnel.

Don’t assume that it is safe to touch adjacent equipment, machinery, pipes, cabinets or even

the earth itself. Adjacent equipment may not have been properly wired or grounded, may be

defective or may have one or more loose system grounds. Measure between the case of a

questionable piece of equipment and its earth ground for voltage. If a voltage is present,

something is wrong.

AC powered equipment with a conductive case should have the case grounded. If you don’t

see a chassis ground wire, don’t assume that it is safe to touch this equipment. If you notice

that equipment has been grounded to pipes, conduit, structural steel, etc., you should be

leery. Note: AWWA’s policy on grounding of electric circuits on water pipes states,

“The American Water Works Association (AWWA) opposes the grounding of

electrical systems to pipe systems conveying water to the customer’s premises….”

Be sure that the voltage between any two points in the instrument system is less than the

stand-off voltage. Exceeding the stand-off voltage will cause damage to the instrument and

will cause the instrument to fail.

Section 3 - Grounding & Isolation Page 3-1 S1400

Section 3 - Grounding & Isolation

3.1 POWER & GROUND SYSTEMS

Series 33XX DPCs and RTUs (3305, 3310, 3330, 3331, 3332, 3335) utilize DC power

systems. With the exception of some 3305 SAPs, AC power supplies are not provided with

these units. Series 3305 SAP RTUs and 3335 DPCs are provided with a Ground Lug that

accommodates up to a #4 AWG size wire for establishing a connection to Earth Ground.

Table 3-1 provides ground connection information for these units.

Model Gnd. Jumpers Doc. Ref. Location Notes

3305 RTU None CI-3305 - C2 MI/OB Bd.

# 12 AWG Wire

Pin 1 = PCOM (DC Ret.)

Pin 2 = POWER (DC+)

Pin 3 = CHASSIS

3305 SAP None

CI-3305 - Sup 1

Section 2.2.3

Ground Lug

#4 AWG - see Figs.

S1-5, -6

3310 RTU

W1A, W1B

See C3A - topic

Grounding Option

CI-3310 - C3A MFIB Bd.

#14 AWG Wire - see Figs.

3A-1, -2, -4, -6

3330 DPC

3332 RED

W1A, W1B, W1C

See C3A - topic

Grounding Option

CI-3330 - C3A

CI-3330 - C3K

SI Bd.

#14 AWG Wire - see Figs.

3A-1, -2, -3, 3K-7, 3K-8

3331 RIO

3335 DPC

W13A, W13B

See C3A - topic

Grounding Option

CI-3335 - C3L

CI-3335 - C3A

Jumpers = SI Bd.

Wiring = System

Monitor Module

& Ground Lug

# 12 AWG Wire from

SMM - TB1 pin 2 (GND)

to Zero Ref. Point - see

Figs 3A-1, -2, -7, -8, -9

#4 AWG Wire from

Chassis Bonding Bolt to

Power Grid Ground

3.2 IMPORTANCE OF GOOD GROUNDS

Series 33XX DPCs and RTUs are utilized in instrument and control systems that must

operate continually and within their stated accuracy over long periods of time with

minimum attention. Failures resulting from an improperly grounded system can become

costly in terms of lost time and disrupted processes. A properly grounded system will help

prevent electrical shock hazards resulting from contact with live metal surfaces, provide

additional protection of equipment from lightning strikes and power surges, minimize the

effects of electrical noise and power transients, and reduce signal errors caused by ground

wiring loops. Conversely, an improperly grounded system may exhibit a host of problems

that appear to have no relationship to grounding. It is essential that the reader (service

technician) have a good under-standing of this subject to prevent needless troubleshooting

procedures.

WARNING

This device must be installed in accordance with the National

Electrical Code (NEC) ANSI/NEPA-70. Installation in hazardous

locations must also comply with Article 500 of the code.

S1400 Page 3-2 Section 3 - Grounding & Isolation

3.3 TRANSIENTS AND INTERFERENCE

The extensive use of low-power integrated circuitry in modern electronic equipment

requires proper grounding techniques to insure reliable system operation. The following

checklist will help identify some critical areas:

1. All instrumentation devices at the site should be checked so that no potential greater

than the standoff voltage can exist within or between devices.

2. To minimize outside signal interference and provide equipment protection from

lightning or transients, the earth ground at the site must be tested to insure that its

impedance measures less than 10 ohms at 7 MHz. This qualification is essential since a

transient potential or an interference signal at the instrument site can vary over the

entire electromagnetic spectrum from DC to several hundred MHz.

Note that transients can be produced through natural phenomena and man-made

conditions. Natural transients may result from lightning (7-14 MHz), static (many

frequencies), and wind (DC charge and static). Man-made transients can result

from defective light bulbs or electrical appliances, sudden electrical load shifts,

inductive load surges, arcing contacts and poor AC power connections.

3. If radio frequency (RF) interference is present at the input of an instrument, observe if

it has a consistent or irregular pattern. Constant interference can come from

commercial radio stations, while irregular interference

can come from private stations.

Although shielding and grounding will eliminate or minimize most cases of RF

interference, obstinate cases may require attenuation filters.

RF interference can also be caused by power companies that apply modulated RF

signals to power lines to communicate data. Other RF noise sources include digital

clocks, computers, relay contacts, motors transformers, switches, arc welders, etc.

3.4 EARTH GROUND CONNECTIONS

To properly ground a DPC or RTU unit, the units Chassis Ground (post or terminal) must

ultimately be connected to a known good Earth Ground. Observe recommendations

provided in topics Establishing a Good Earth Ground and Ground Wire Considerations

3.4.1 Establishing a Good Earth Ground

A common misconception of a ground is that it consists of nothing more than a metal pipe

driven into the soil. While such a ground may function for some applications, it will often

not be suitable for a complex system of sophisticated electronic equipment. Conditions such

as soil type, composition and moisture will all have a bearing on ground reliability.

A basic ground consists of a 3/4-inch diameter rod with a minimum 8-foot length driven into

conductive earth to a depth of about 7-feet as shown in Figure 3-1. Number 3 or 4 AWG

solid copper wire should be used for the ground wire. The end of the wire should be clean,

free of any coating and fastened to the rod with a clamp. This ground connection should be

covered or coated to protect it from the weather and the environment.

Section 3 - Grounding & Isolation Page 3-3 S1400

Figure 3-1 - Basic Ground Rod Installation

Figure 3-2 - Overhead Map of Ground Bed for Gas Metering Station

S1400 Page 3-4 Section 3 - Grounding & Isolation

3.4.1.1 Soil Conditions

Before installing a ground rod, the soil type and moisture content should be analyzed.

Ideally, the soil should be moist and moderately packed throughout to the depth of the

ground rod. However, some soils will exhibit less than ideal conditions and will require

extra attention.

Soil types can be placed into two general categories with respect to establishing and

maintaining a good earth ground, i.e., ‘Good Soil’ and ‘Poor Soil.’

To be a good conductor, soil must contain some moisture and free ions (from salts in the

soil). In very rainy areas, the salts may be washed out of the soil. In very sandy or arid area

the soil may be to dry and/or salt free to a good conductor. If salt is lacking add rock salt

(NaCl); if the soil is dry add calcium chloride (CaCl

3.4.1.2 Soil Types: Good Poor

Damp Loam Back Fill

Salty Soil or Sand Dry Soil

Farm Land Sand Washed by a Lot of Rain

Dry Sand (Desert)

Rocky Soil

Ground Beds must always be tested for conductivity prior to being placed into service. A

brief description of ground bed testing in ‘Good Soil’ and ‘Poor Soil’ is provided herein.

Details on this test are described in the National Electrical Code Handbook. Once a reliable

ground has been established, it should be tested on a regular basis to preserve system

integrity.

Figure 3-3 - Basic Ground Bed Soil Test Setup

Section 3 - Grounding & Isolation Page 3-5 S1400

Figure 3-3 shows the test setup for ‘Good Soil’ conditions. If the Megger* reads less than 5

ohms, the ground is good. The lower the resistance, the better the earth ground. If the

Megger reads more than 10 ohms, the ground is considered ‘poor.’ If a poor ground is

indicated, one or more additional ground rods connected 10 feet from the main ground rod

should be driven into the soil and interconnected via bare AWG 0000 copper wire and 1” x

¼-20 cable clamps as illustrated in Figure 3-4). * Note: Megger is a Trademark of the

Biddle Instrument Co. (now owned by AVO International). Other devices that

may be used to test ground resistance are “Viboground”; Associated Research,

Inc., “Groundmeter”; Industrial Instruments, Inc., and “Ground-ohmer”; Herman

H. Sticht Co., Inc.

If the Megger still reads more than 10 ohms, mix a generous amount of cooking salt, ice

cream salt or rock salt with water and then pour about 2.5 to 5 gallons of this solution

around each rod (including the test rods). Wait 15 minutes and re-test the soil. If the test

fails, the soil is poor and a ‘Poor Soil Ground Bed’ will have to be constructed.

Figure 3-4 - Basic Ground Bed Soil Test Setup with Additional Ground Rods

Figure 3-5 shows a typical Poor Soil Ground Bed Electrode. A Poor Soil Ground Bed will

typically consists of four or more 10-foot long electrodes stacked vertically and separated by

earth. Figure 3-6 shows the construction of a Poor Soil Ground Bed. For some poor soil

sites, the ground bed will be constructed of many layers of ‘Capacitive Couplings’ as

illustrated. In extremely poor soil sites one or more 3’ by 3’ copper plates (12 gauge or 1/16”

thick) will have to be buried in place of the electrodes.

Figure 3-5 - Ground Electrode Construction for Poor Soil Conditions

S1400 Page 3-6 Section 3 - Grounding & Isolation

3.4.1.3 Dry, Sandy or Rocky Soil

Very dry soil will not provide enough free ions for good conductance and a single ground rod

will not be effective. A buried counterpoise or copper screen is recommended for these

situations. It will be necessary to keep the soil moist through regular applications of water.

Sandy soil, either wet or dry, may have had its soluble salts leached out by rain water,

thereby reducing conductivity of the ground. High currents from lightning strikes could also

melt sand and cause glass to form around the ground rod, rendering it ineffective. A buried

counterpoise or copper screen is preferred for these installations along with regular

applications of salt water.

Rocky soil can pose many grounding problems. A counterpoise or copper plate will probably

be required. Constructing a trench at the grounding site and mixing the fill with a

hygroscopic salt such as calcium chloride may help for a time. Soaking the trench with

water on a regular basis will maintain conductivity.

Figure 3-6 - Poor Soil Ground Bed Construction Diagram

Section 3 - Grounding & Isolation Page 3-7 S1400

3.5 GENERAL RECOMMENDATIONS

When wiring equipment into a system, the electrical conduit must have a diameter that will

accommodate the desired number of wires. The cross- sectional area of the conduit should

be large enough to allow the wires to be pulled through without excessive tightness or

binding. A conduit that is too tight can shred insulation, damage wiring, and result in

possible opens, shorts, or intermittent effects. Such conditions are often difficult to trace

because the defect is concealed inside the conduit.

3.5.1 Noise and Signal Errors

Noise and signal errors are often the result of poor wiring and grounding practices. Some

common problem areas are listed as follows:

o Shielding AIs and AOs. Very often analog DC signal leads must run parallel to

wires radiating AC fields, pulse information, or switching transients. Due to

inductive and capacitive pickup, some of this information can leak into an analog I/O

and cause peculiar effects in the control systems. To minimize or eliminate this

problem, the use of insulated and shielded, twisted lead pairs is recommended

between the external devices (transmitters, sensors, etc.) and the instrument inputs

(controllers, recorders, etc.).

The shields of each analog signal source should only be grounded at the input of the

in-strument. In some equipment, the shield will connect to the instrument chassis.

In other equipment, a "shield" terminal will be provided with several grounding

options. The user should refer to the instrument manual and follow grounding

recom-mendations.

o Common Returns. The use of a single "common" return wire for two or more input

signals is not recommended. This approach may introduce system ground loops that

cause erroneous readings at the instrument. Shielded transmitter or sensor wires

should be grounded at the input of the instrument, or connected to a shield terminal

(where provided) to prevent "sneak" ground paths.

o Discrete Outputs. Instruments provided with bi-state discrete outputs perform

functions such as control switching, alarm switching or pulse duration com-

munications. These outputs are furnished as either open collector or relay contact

outputs that operate at low power levels. While these levels are sufficient to operate

many devices, some will require much higher power levels. The use of external

amplifiers or repeating relays to drive end devices will prevent output overload and

add to the reliability of the system.

o Placement of Wiring. The dressing or physical placement of wiring requires close

scrutiny. Cables inside cabinets should be neatly secured at regular intervals.

Cables running between cabinets at different locations should be placed in conduits.

The cable length should allow sufficient slack for routine operational checks and

maintenance of the equipment. Wiring from input signal circuits and power circuits

should be separated as much as possible to minimize noise and transient pickup.

Power and signal leads should be run in separate conduit to minimize inductive

pickup.

S1400 Page 3-8 Section 3 - Grounding & Isolation

o Terminal Lugs. The use of crimp-type terminal lugs as connections for screw

terminals should be avoided. Terminal lugs, in many industrial climates, can be

affected by hidden corrosion. It is preferable to tin the wire end with solder and loop

it around the terminal screw. The screw should be tightened sufficiently to hold the

lead in place but not excessively so that the lead is sheared or the screw is stripped.

Equipment furnished with compression-type terminals includes an opening for

inserting tinned ends.

3.5.2 General Considerations

The following considerations are provided for the installation of system grounds:

i Size of ground wire (running to Earth Ground should be #4 AWG. It is recommended

that stranded copper wire is used for this application and that the length should be as

short as possible.

i This ground wire should be clamped or brazed to the Ground Bed Conductor (that is

typically a stranded copper AWG 0000 cable installed vertically or horizontally).

i The wire ends should be tinned with solder prior to installation.

i The ground wire should be run such that any routing bend in the cable has a

minimum radius of 12-inches below ground and 8-inches above ground.

The units Earth Ground Cable should be clamped to an exposed Ground Rod or to an AWG

0000 stranded copper Ground Cable that in turn should be connected to either an Earth

Ground Rod or Earth Ground Bed. Both ends of the units Earth Ground Cable must be free

of any coating such as paint or insulated covering as well as any oxidation. The connecting

point of the Ground Rod or AWG 0000 Ground Cable must also be free of any coating and

free of oxidation. Once the ground connection has been established (at either the Ground

Rod or Ground Cable) it should be covered or coated to protect it from the environment.

3.5.3 Other Grounding Considerations

Instrument enclosures, measuring devices, metal process vats, metal piping, and other

associated mechanical and electrical devices should all be grounded. The method of

grounding an instrument rack is shown in Figure 3-6. In this application the ground lead

typically attaches to a ground bus that is common to all equipment in the rack.

For applications employing equipment that communicates over telephone lines, a lightning

arrester Must Be provided. For indoor equipment the lightning arrester must be installed

at the point where the communication line enters the building as shown in Figure 3-7. The

ground terminal of this arrester must connect to a ground rod and/or a buried ground bed.

Applications that use transmitters or transducers require grounding and shielding. In

Figure 3-8, the ground conductor feeds through the electrical conduit and connects to the

ground screw of the transmitter even though the support pipe is grounded. However, if the

transmitter uses shielded wiring for its signal output, the shield should not be grounded at

the transmitter. For maximum signal accuracy, the shield should only be grounded at one

point in the system, typically at the input of the associated equipment.

Section 3 - Grounding & Isolation Page 3-9 S1400

Figure 3-6 - Grounding of Equipment Housing

Figure 3-7 - Grounding of Phone Line

S1400 Page 3-10 Section 3 - Grounding & Isolation

Figure 3-8 - Grounding of Transmitter

Figure 3-9 - Grounding of Thermometer Well in Gas Line

Section 3 - Grounding & Isolation Page 3-11 S1400

Gas lines also require special grounding considerations. If a gas meter run includes a

thermocouple or RTD sensor installed in a thermowell, the well (not the sensor) must be

connected to a gas discharge-type lightning arrestor as shown in Figure 3-9. A copper braid,

brazed to the thermal well, is dressed into a smooth curve and connected to the arrestor as

shown. The curve is necessary to minimize arcing caused by lightning strikes or high static

surges. The path from the lightning arrestor to the ground bed should also be smooth and

free from sharp bends for the same reason.

The ac power required to operate a system typically includes a service transformer located

at the street and a main breaker box and rate meter assembly at the building as shown in

Figure 3-10. The service transformer is grounded at the company's pole, while the breaker

box is grounded at the building. A lightning arrestor should be included at the breaker box

in each phase of the AC line, and each arrestor should be grounded accordingly.

Figure 3-10 - AC Power Grounding System

S1400 Page 3-12 Section 3 - Grounding & Isolation

3.6 GROUNDING TECHNIQUES FOR SERIES 33XX SYSTEMS

When installing a system that includes a number of Bristol Babcock, Series 33XX

Distributed Process Controllers (DPCs), it is essential to follow the procedures set forth by

the National Electrical Code (NEC) to minimize risk of equipment damage and electrical

shock.

WARNING

Electrically powered equipment must be properly grounded to

protect users from electrical shock and injury. All such devices

must be installed, wired, and grounded in accordance with the

National Electrical Code (NEC).

Series 33XX DPCs employ a power grid ground terminal (CHASSIS) and an instrument

ground terminal (24VRET) that connects to the "zero reference point" of the system.

Improper grounding of these terminals can produce multiple ground paths throughout the

system and result in increased noise pickup and signal offset errors. If more information is

required on this subject, the reader should refer to the publications cited at the end of this

document.

The examples that follow describe the grounding techniques for several types of Bristol

Babcock systems employing DPCs. Refer to the system description that is closest to your

application.

3.6.1 Several DPCs Mounted in Metal Cabinet with Power Supply

A small system can consist of one or more DPCs mounted in a single metal cabinet or rack

with a power supply. A power wiring diagram for this arrangement is shown in the example

of Figure 8. The following installation procedures apply:

1. Instrument Ground. The instrument ground of the DPCs (24VRET terminal of each

DPC) must connect to a terminal block within the cabinet that is electrically isolated

from the cabinet frame. This terminal block must provide termination for all in-

strument grounds and include termination for a multistranded, insulated, #4 gauge

wire (or greater). This wire, which will connect to the "zero reference point" of the

facility, must be run through metal conduit (pipe). Only the #4 wire will be contained

in this conduit. The conduit must also be connected by bonding strap to the cabinet

and facility frame as described in the NEC.

2. Setting DPC Power Jumpers. If the DPC is a Model 3335 or 3310, jumpers W1A and

W1B on the System Interconnect Board must be removed to isolate the chassis

connection from the 24VRET connection (see Figure 3-11). If it is a Model 3330,

jumpers W1A, W1B and W1C on the System Interconnect Board must be removed.

Series 3308 Gas Flow Computers or Correctors, if used with these systems, provide an

isolated instrument ground without setting jumpers.

3. AC Power Source. The 24 Vdc power supply requires a 120 Vac power source. The ac

power terminals of this supply are identified in Figure 3-11. The 120 Vac wiring for

this supply must be contained in cable trays along with the power grid grounding

wire. Figure 3-12 illustrates the cable tray layout and grounding points of a typical

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Remote Automation Solutions Bristol 33XX Site Considerations for Equipment Owner's manual

Remote Automation Solutions Bristol 33XX Site Considerations for Equipment Owner's manual

Remote Automation Solutions Bristol 33XX Site Considerations for Equipment Owner's manual

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